The production of N2O in Douglas fir litter as affected by anoxic conditions within litter particles and pores

The development of anoxic conditions in forest litter and the relation with nitrous oxide (N2O) production and emission rates are not completely understood. Water content is an important factor in the regulation of N2O production due to its effect on the development of anoxic conditions. A combination of simulation modeling and incubation experiments was used to study (1) O2 concentrations in water and organic matter at various water saturation fractions of inter-particle pores in Douglas fir litter (F2 horizon), (2) the relationship between N2O production and moisture content of litter and (3) to test whether diffusion constraints of nitrate (NO3−) could have explained measured N2O production rates within litter fragments. Model simulations showed that the occurrence of high N2O production rates in samples with extremely high water contents coincided with the development of anoxic conditions in water-filled inter-particle pores. Measured N2O production rates started to increase exponentially after 1–2 days in glucose-amended samples, during which substantial microbial growth was established. For these latter samples model simulations showed that the increase in O2 consumption due to microbial growth lead to anoxic conditions in water-filled pores at locations which were far from the O2 saturated air-filled pores. It was concluded that anoxic conditions in water-filled pores was the crucial factor for the development of high N2O production rates. Diffusion limitation of NO3− and glucose were estimated to be negligible in the highly fragmented litter material used. The occurrence of diffusion limitation depended on litter particle size, the NO3− reduction potential and the NO3− concentration. Therefore, diffusion limitation together with N2O production in litter cannot be neglected under field conditions with a low NO3− concentration or a high NO3− reduction potential.